1. Field of the Invention
This invention relates to optical smoke detectors which employ detectors reponsive to both light obscuration and light scatter.
Light obscuration smoke detectors depend upon measurement of the degree of obscuration of a detector resulting from the presence of smoke between the detector and a light source. The light obscuration method of smoke detection is highly accurate and is used as the standard against which ionization and light scatter detectors are measured. Typically such a smoke detector comprises a chamber with an emitter the output of which is directed to a sensor at its opposite end. The chamber is provided with light trapped openings for admitting smoke. The presence of smoke in the direct optical pathway between the emitter and the sensor results in absorption of light, thus reducing the output of the sensor and, through suitable electronics, actuating an alarm.
Light scatter smoke detectors depend upon the back scatter or forward scatter of light, the so-called Tyndall effect, which results from the presence of smoke in a light beam. Typically, a light emitter such as a diode illuminates the inside of a smoke detector chamber while a sensor, the axis of sensitivity of which is directed at an angle to that of the emitter axis, monitors the chamber interior. The presence of smoke generates a signal in the sensor which is received by an amplifier and comparator, the latter having a threshold level. The presence of smoke increases the output of the sensor; when the threshold level is exceeded the alarm is actuated.
Because smoke detectors frequently are located in environments where airborne dust is present, it is necessary that the operation of the detector be effectively immune to the accumulation of dust and dirt within the chamber.
An important cause of malfunctions, such as false alarms, in many smoke detectors of the light scatter type is the presence of dust within the smoke chamber. The dust layer accumulating on the side, top or bottom walls has a higher reflectivity than that of the conventional black walls of the chamber; hence, stray light from the light source striking such dusty walls results in increased light reaching the light detector which interprets this increase as indicating the presence of smoke and consequently energizes the alarm. In the present invention substantially all of the light source is reflected back on itself, with only a small portion spilling over the edge of the reflector and onto the walls of the smoke chamber.
Smoke detectors of the obscuration type also tend to malfunction when dust accumulates within the smoke chamber. The signal received by the light detector is the sum of the light received directly from the light source less that absorbed by any smoke that may be present plus that reflected from the chamber walls. The accumulation of dust on the walls of the obscuration type of detector increases the level of this reflected light and thus acts as a significant secondary light source which, in the presence of a given level of smoke, counteracts the light attenuation induced by the smoke, increasing the level of smoke intensity tow hich it is intended to respond and thereby resulting in a potentiall dangerous delay in activating the alarm. In the present invention substantially all of the light from the light source is directed onto the light detector with almost no light striking the walls of the smoke chamber.
2. Description of Prior Art
The concept of reflecting light from a smoke detector light source back on itself has been shown in U.S. Pat. No. 4,221,485 to R. Schulze. In this smoke chamber, a spherical reflector receives light from an LED (light emitting diode) which is centered on a planar photodetector. In the absence of smoke most of the light from the LED is reflected back on itself without falling on the photodetector. However, even a small misalignment of the mirror during manufacture or as a result of conditions during use would divert the reflected light from the center of the LED and partially onto the photodetector sending the device into a alarm condition.
A smoke detector which allegedly responds to smoke in both the absorption mode and the light-scatter mode is shown in U.S. Pat. No. 3,922,655 to D. Lecuyer. Here a dual photo cell receives light from a single source. The function in the scatter mode is in accordance with general practice: one part of the photocell is directed at approximately a right angle to the axis of the light source and receives light scattered by the smoke. In the so-called absorption mode, a second part of the photocell receives light reflected from the walls of the smoke chamber via a mirror. The second part of the photocell is not optically aligned with the light source and does not receive light directly from the latter, a necessary condition for absorption mode function. Instead the second part of the cell actually receives light scattered by smoke in the chamber; in effect, the Lecuyer showing is in fact a combination of two light-scatter detectors.